1. IN625
Steel
PWHT F22 BS1
7
Objectives
• Verify the success of the post weld heat treatment (PWHT) at lowering
the hardness of the heat affected zone (HAZ) to meet NACE
requirements (< 250 VHN).
• Explore the effect of different substrates and weld bead sequences.
Results: Micro-Hardness Maps
Methods
• Eight hot-wire gas tungsten arc welded (GTAW) overlays were
prepared as they would be for subsea service.
• Samples were cut from the DMW interface (red boxes in 2 & 3),
mounted and prepared for optical microscopy.
• Chromic acid was used as the etchant for the 625 Nickel based alloy,
and 10% nital was used to etch the base steel forging.
• After pictures were taken and analyzed, the etchant was polished off
and Micro hardness maps were done over the interface.
• From the hardness data, hardness distribution by value graphs were
made to display the hardness trends across the DMW interface.
• PWHT
• Solved the HAZ hardness issue (from 370 VHN to 250 VHN)
• Caused carbon migration to the fusion boundary raising the hardness
(from 300 VHN to 450 VHN)
• The base metal hardness may have been lowered below an
expectable amount (from 270 VHN to 170 VHN)
• Substrate
• 8630 HAZ had higher hardness than F22 in all combinations (~25
VHN higher)
• BS (Bead sequence)
• BS1 has higher hardness values than BS3 in all material combinations
• BS1 was more susceptible to LOF defects
There have been several failures of Dissimilar metal welded
(DMW) pipes in subsea service. These DMW pipe joints are typically
done in three steps. First a multi-pass butter layer (of Inconel 625) is
added to both of the joint faces in one of two ways; in bead sequence
one (BS1) welds are layered normal to the fusion boundary, in bead
sequence three (BS3) welds are layered parallel to the fusion boundary.
The base forgings (either F22 or 8630 steel) are then post-weld heat
treated (PWHT) to temper the heat affected zone (HAZ). Finally a closure
weld is completed in the field using the same welding consumable as in
the buttering layer.
The butter layer applied is thick enough such that the HAZ of the
closure weld does not reach the base metal, and thus not critically
hardening the base forging.
Subsea pipes are cathodically protected to prevent corrosion. While
this keeps the pipes from rusting it also evolves atomic hydrogen at the
surface of the pipes and leads to hydrogen ingress into the pipes. The
failures mentioned earlier have been attributed to high hardness and this
buildup of hydrogen in the DMW fusion zone.
Acknowledgements
0%
1%
2%
3%
4%
5%
6%
7%
8%
150 250 350 450
PercentofIndents
Hardness (VHN)
8630 BS1
8630 BS3
0%
1%
2%
3%
4%
5%
6%
150 250 350 450
PercentofIndents
Hardness (VHN)
F22 BS1
F22 BS3
0%
1%
2%
3%
4%
5%
6%
7%
8%
150 250 350 450
PercentofIndents
Hardness (VHN)
PWHT 8630 BS1
PWHT 8630 BS3
0%
1%
2%
3%
4%
5%
6%
7%
8%
150 250 350 450
PercentofIndents
Hardness (VHN)
PWHT F22 BS1
PWHT F22 BS3
IN625
SteelF22 BS3
PWHT 8630 BS1 PWHT F22 BS1
Post weld heat treated (PWHT)
reduced the hardness in the heat
affected zone (HAZ) (4 & 5).
PWHT caused carbon to diffuse
to the fusion boundary which
increased the hardness there (6).
PWHT lowered
the hardness
of the base
metal to below
250 VHN (4 &
6). 8630 had
higher
hardness than
F22 in all
combinations
(4 & 6).
Schematic of a DMW joint
625
BS3
625
BS1
625
Steel
Steel
Step One
Final Step
IN625
SteelPWHT 8630 BS1
Results: Hardness Distribution by value Graphs
Bead sequence one (BS1) was harder and hence more brittle in all
material combinations than bead sequence three (BS3).
Results: Visual inspection
Bead sequence one (BS1)
produces lack of fusion (LOF)
defects at the dissimilar metal
weld (DMW) interface in all
material combinations.
Conclusion
Introduction